Spatial frequency-coherence

in an optical system, there's what they call coherence term. its symbolised by sigma. there's a relation between the spatial frequency and this term.
if an optical system has a big sigma it will had then a better resolution? or what?
and whats the relation between the spatial frequency and the resolution???

and whats the relation between the spatial frequency and the resolution???

The maximum resolution you can get fundamentally is the maximum spatial frequency you can capture. The operative term you should keep in mind here is maximum, there a lots of other things that can degrade resolution.